1. Field of the Invention
The present invention relates to a method for manufacturing an SOI substrate, and particularly relates to a method for manufacturing an SOI substrate in which a silicon film is formed on one main surface of a transparent insulating substrate.
2. Description of Related Art
In order to achieve higher performance of semiconductor devices, attention has been focused on SOI (silicon on insulator) substrates in recent years. Additionally, some types of SOI substrates such as a silicon on quartz (SOQ) substrate and a silicon on glass (SOG) substrate which have support substrates (handle wafers) made of non-silicon materials are also expected to be applied to TFT-LCDs, high-frequency (RF) devices, other MEMS products, and so forth (for example, see JP2006-324530A).
There has been proposed a method for manufacturing the aforementioned SOQ substrate or the like, by using, for example, a silicon substrate for a donor wafer and a quartz substrate for a handle wafer, and bonding these different substrates together. Since the quartz substrate is transparent in the above bonded substrate, the bonded substrate may have different problems in process and evaluation from the problems of normal SOI substrates manufactured by bonding a silicon substrate to another silicon substrate.
One of such problems is that, when being conveyed on equipment, an SOI substrate including a silicon film formed on a transparent insulating substrate such as an SOQ substrate (hereinafter, sometimes abbreviated as a transparent SOI substrate) is unlikely to be recognized by an optical sensor for recognizing substrates, for example.
Meanwhile, a sandblasting method is used for applying a fogging treatment (frost treatment) to a substrate, component and so forth based on SiO2 such as glass, quartz in some cases. In this method, a to-be-treated surface is blasted with alumina or silica fine particles to be roughened. The method is widely applied for diverse applications.
However, in the fields of electronic materials and devices, foggy surfaces formed by such a method have several problems. One of them is a particle (foreign matter) problem. This problem is caused by a sandblast powder remaining on a treated surface; dust generated from an acute portion, a crack and a damaged portion of a roughened surface; and so forth. These problems cannot be solved by normal cleaning in many cases. Moreover, a problem such as metal contamination attributable to this foreign matter is also serious in the field of electronic materials.
This particle problem can be critical particularly when a product treated by this fogging treatment is used in the field of semiconductor. For example, a quartz boat or the like for a wafer to be used in a diffusion furnace or the like is sometimes subjected to a fogging treatment to prevent the wafer from closely adhering to a groove for holding the wafer. In this case, since the quartz boat or the like undergoes a high-temperature process, certain measurement needs to be taken against metal contamination in addition to particles. Additionally, there is a problem that particles on transparent substrates such as those of SOQ (Silicon on Quartz) and SOG (Silicon on Glass) significantly increase in amount when back surfaces of the transparent substrates are subjected to the fogging treatment for making the transparent substrates recognizable by substrate-recognition sensors of various apparatuses.
To remove the particles after such a sandblasting process, a cleaning step is performed. In this cleaning step, HF cleaning has been employed, for example. The HF cleaning, however, has a problem of making the particle level rather worse. This is because the HF cleaning activates the surface of a glass or the like; moreover, fine pieces of the glass or the like released during the cleaning reattach to the surface (for example, see Science of Silicon, Chapter 4, Section 4, Realize Science & Engineering Center Co., Ltd.). Furthermore, if the cleaning is performed for a long period with high concentration HF to remove the particles, the surface subjected to the fogging treatment is made excessively smooth, thereby decreasing the effect of roughing the surface.